I got my aerial sorted out now. You guys gave me some good suggestions, but I thought I'd give an ebay cheapie a try first.
The aerial I went for is this one:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=160601911181&ssPageName=STRK:MEWNX:IT
Total cost was £4.84 including postage, I bought it on Sunday and it arrived today. Here it is installed:
Opinions may differ but I think it looks really good, and importantly for me, wont be easily bent and yanked by any passing vandals. I've tested the reception and it's better than with the standard aerial, really clear.
Just one question - does this officially count as my first modification?